Around 1 hr and 25 mins into the movie, Stallone finds the 3rd and last suitcase. However, look carefully when he first spots the case and also when you get a rear-view shot of him stuffing his backpack with the cash. You can see another suitcase just above his head slightly to the left.
It isn't another suitcase...it's the top of the suitcase Sly is emptying. Take another look at the movie. when he first gets to the case he scrapes snow off the money...but there is no top to the case.